Thermodynamic model of the oxidation of Ln-doped UO2

Abstract

Ln-doped UO₂ is often considered as a model system of spent nuclear fuel (SNF) helping to reveal effects of fission and activation products on its chemical stability. Comparing thermodynamics of UO₂-UO₃ and LnO_1.5-UO₂-UO₃ systems provides a means to understand the phenomenon of an increased resistivity of Ln-doped UO₂ to oxidation in air relative to pure UO₂. Here a thermodynamic model is developed and is applied to investigate detailed phase changes occurring along the oxidation of Ln-doped fluorite to U₃O₈. The study proposes that an enhanced resistivity to oxidation of Ln-doped UO₂ is likely caused by a thermodynamically driven partitioning of Ln between a fluorite-type phase and a U₃O₈ polymorph, which at ambient temperatures becomes hindered by slow diffusion.

Figure 1

Model fit to the experimental data on ${\mathrm{\Delta }G}_{{\mathrm{O}}_2}=\mathit{RT}\ln (P_{{\mathrm{O}}_2}/P^0)$ vs. $\delta =x/2$ for pure UO₂. The experimental data are from Lindemer & Sutton, Nakamura & Fujino and Saito.

Figure 2

Model fit to the experimental data on ${\mathrm{\Delta }G}_{{\mathrm{O}}_2}=\mathit{RT}\ln (P_{{\mathrm{O}}_2}/P^0)$ vs. $\delta =(x-y)/2$ for the system GdO_1.5-UO₂-UO₃. The experimental data are from Lindemer & Sutton. Note, that δ can be written as δ = (x − y)/2, as it is a function of the mole fractions, x and y, of the UO_2.5 and LnO_1.5 endmembers. δ can be also evaluated as δ = O/M − 2. A negative/positive deviation from δ = 0 implies the presence of either oxygen vacancies or oxygen interstitials. Note also, that δ characterizes non-stoichiometry only of a mono-phase fluorite, while O/M − 2 is also applicable to a poly-phase system. ${\mathrm{\Delta }G}_{{\mathrm{O}}_2}$ is a convenient function to visualize effects of the temperature and/or the partial pressure of O₂ on the chemical potential of O₂, while the dimensionless quantity $\log (P_{{\mathrm{O}}_2}/P^0)$ is more convenient when the temperature and the pressure effects need to be distinguished. Both quantities are used throughout the text.

Figure 3

Model fit to the experimental data on $\log (P_{{\mathrm{O}}_2}/P^0)$ vs. $O/M-2$ for pure UO₂. The experimental data are from Saito . Solid lines correspond to the equilibrium with the hexagonal polymorph of U₃O₈. The dashed line corresponds to the orthorhombic α-U₃O₈.

Figure 4

Model fit to the experimental data on $\log (P_{{\mathrm{O}}_2}/P^0)$ vs. $O/M-2$ for UO₂ doped with 4.8 mol % of LaO_1.5. The experimental data are from Stadlbauer et al. . Solid lines correspond to the equilibrium with the hexagonal polymorph of M₃O₈. The dashed line corresponds to the orthorhombic α-M₃O₈.

Figure 5

Predicted evolution of composition of phases in a sample containing 4.8 mol % of LaO_1.5 in the process of equilibrium oxidation at 673 and 873 K. Filled and empty symbols correspond to 873 K and 673 K isotherms, respectively.

Figure 6

Fractions of phases in a sample containing 4.8 mol % of LaO_1.5 in the process of equilibrium oxidation at 673 K. Symbols are the same as in Fig. 5.

Figure 7

Variation of the lattice parameter in UO₂-NdO_1.5 solid solutions predicted from the thermodynamic model. Solid lines are the 1123 K isotherms computed at different values of $\log (P_{{\mathrm{O}}_2}/P^0)$ such that the pressure variation approximately covers the range of redox conditions in the data of Wadier . Dashed lines (red online) are the 1373 and 1673 K isotherms computed at $\log \left(P_{{\mathrm{O}}_2},/,P^0\right)=0$. These isotherms correspond to the synthesis conditions in the study of Keller & Boroujerdi. The other experimental data are from Lee et al., Fukushima et al., Ohmichi et al. and Une & Oguma.